The widespread use of portable power tools in the construction, lumbering and consumer markets has emphasized the need for effective user safeguards. Lighter weight, reduced vibration and higher power have resulted in very efficient cutting tools that are widely used by skilled, semiskilled and unskilled operators. As with any power driven machine they represent a potential hazard to the user if suitable safeguards are not provided. Thus, for example in the case of chain saws which are becoming more and more widely used by householders as well as by construction workers and woodsman, a potential hazard is presented by the phenomenon known as "kickback." A kickback can be produced by a chain saw when the cutting elements on the chain momentarily are caught or seized by the work material. The kinetic energy of the chain normally used to remove chips of the material being cut is suddenly transferred to the chain bar to which it imparts a force causing the chain bar to kick up toward the operator. The magnitude of the kickback force is related to the speed of the chain and the nature of engagement of the chain with the work material. In order to avoid possible injury to the operator it is desirable to provide means for stopping the chain before the chain bar can come into contact with the operator. For this purpose it has been proposed to provide a chain saw with a brake or other device for stopping the chain in the event of a kickback or other hazardous mishap. However, because of the inertia of the engine and the drive line between the engine and the cutting chain, it is difficult to stop the chain in a sufficiently short period of time.
It has also been proposed to provide a quick disconnect coupling between the engine and the sprocket by which the chain is driven. However, the couplings sofar proposed have left much to be desired in constructional and operational characteristics. By way of example one proposed coupling has the undesirable characteristic that after the coupling has disengaged at operating speed there is a possibility of its reengagement when the engine slows down as a result of the operator releasing the usual throttle-operating trigger. Another coupling heretofore proposed is undesirably complex in construction and operation and requires two manual operations to reset the coupling, one to reengage the coupling and the other to reset the actuating mechanism.